Although many thermoplastic resins have been found to be injection moldable over a broad range of conditions, only certain thermoplastic resins are suitable for blow molding. This is due to the unique demand placed on a thermoplastic material by the blow molding operation. In one aspect of the conventional blow molding operation, a cylindrically shaped parison, or preform, of a heat softened thermoplastic material is extruded vertically downward until the parison has attained a preselected length. A split cavity mold is then closed around the parison, pinching its walls together at least at one end, and optionally at intermediate locations and at the other end as well. Compressed gas is injected into the parison, either from one of the ends or through one or more blow pins inserted in its side, to inflate the parison against the sides of the mold in a fashion similar to the expansion of a balloon. Contact of the thermoplastic material with the mold causes it to cool and harden in the shape of the mold.
Because a parison is formed as a single, integral piece of thermoplastic material, and the inflating medium acts from its interior to shape it with outward pressure, blow molding makes possible the fabrication of "hollow" articles, wherein two or more walls define or contain a completely enclosed space, or a series of such spaces. These articles range from as simple a shape as a shampoo bottle to a complex article such as an extensively contoured automotive instrument panel containing multiple interior channels which function as ducts for wiring or for a heating/ventilating/air conditioning system. However, the successful use of a particular thermoplastic resin in a blow molding operation, as described above, is dependent to a great extent on certain physical characteristics of the heat softened polymeric resin or material. A low level of melt strength in the material from which the parison is extruded can limit the length and diameter of the parison, and thus the size and wall thickness of the object being molded.
The use of fibril-forming polytetrafluoroethylene ("PTFE") to increase the melt viscosity and elasticity of high molecular weight organic polymers for use in melt extrusion and thermoforming is set forth in Busse, U.S. Pat. No. 3,005,795. However, no mention is made therein of the blow molding process, or of the use of non-fibril-forming PTFE or other fluorine-containing olefin polymers or copolymers for the purpose of improving the melt properties of an organic polymer. Nor is any suggestion present therein as to the use of a fluorine-containing olefin polymer or copolymer in conjunction with a blend of two or more organic polymers.
It would accordingly be desirable to have methods, and to have compositions useful in such methods, for employing a variety of fluorinated olefinic polymers or copolymers to increase the melt strength of a variety of thermoplastic polymeric materials or resins to be used in a blow molding operation, thereby improving the quality of articles blow molded from said compositions.